Presently, most users expect power converters with multiple protection functions, low cost, high density and high reliability, which will meet needs of users. Protection functions include such as prevention of input under-voltage, input over-voltage, over-temperature, output over-voltage and output over-current (short circuit) and the like. The most point concerned by a user is output over-voltage protection and output over-current protection (i.e., short circuit protection), both of which have mostly significant influence on final user. It will bring great loss to final users in case that these two protection functions fail to work. Nowadays, almost all output over-voltage protection mechanisms employ optical coupling control. As a result, this will not only increase cost, but also result in destruction of power or device in case that feedback optical coupling and over-voltage protection optical coupling work abnormally during test process or used by a user. Among power failures, optical coupling failure has the most likelihood to occur.
1. Output over-voltage protection: A load will be protected if over-voltage occurs at the output end, and by now the following protection types are often taken:
1) Locking type in which a module will be locked if over-voltage occurs at the output end, and in this situation, the output end will not work until power is restarted (sometimes restart will be enabled only after the internal capacitor of the power is completely discharged). This type of protection is inconvenient for users.
2) Voltage restriction type in which the output voltage will not increase any more when the output value reaches a certain value and the output voltage will be locked at a certain value; and resumption will be possible after malfunction is eliminated.
These two kinds of protection both employ dual optical coupling feedback by far and the failure rate of optical coupling is high. When optical coupling of feedback and over-voltage is in failure, the loop will become open, thereby causing the loop out of control and over-voltage losing its function and accordingly, post-devices will subject to vital destruction.
2. Output over-current protection: The power or load will be protected if short circuit or over-load at the output end occurs. The following protection types are frequently utilized:
1) Rated current droop type: This type of protection is widely used in RCC circuit due to characteristics of circuit itself.
By now this type of protection is mostly used in rechargers and the like. However, it is difficult to maintain uniformity during production process and hard to control the process. Moreover, power improvement is also difficult, and reliability is poor, thus this type of protection has its main application in low end users.
2) Current constant type: The control circuit will start when the output current reaches a certain value such that the circuit will enter into protection state.
This type of protection is the best in quality in that protection will functions when the output current becomes a certain value, and hiccup mode (or lock type) will be resumed automatically after cancellation of protection. Current stress resulted during output rectification process will be very small. This mode is extensively used by now, however, to make full use of this mode, and make the input current be extremely small after occurrence of short circuit, additional relevant circuit should be added to keep continuous short circuit or over-current.
3) Power constant type: Generally, the output power is limited at the primary side. As the output current increases, the output voltage will decrease.
This mode operates according to self-protection characteristics of the chip. However, this mode will produce large input and output current when short circuit or over-current presents, thereby resulting in serious current stress for primary MOS and rectifier. In this case, the duty ration will reach the highest limit. Therefore, this mode normally provides no over-current and short circuit protection, and it is almost impossible to maintain continuous short circuit and short circuit time should be very short.
FIG. 1 illustrates voltage-current characteristics chart of the output of the three types of over-current protection in which (1) represents rated current droop type, (2) represents current constant type, while (3) represents power constant type. Among these types, the protection effect of the first and second types is better, while that of the last one is worst.
A block diagram of a conventional power converter is shown in FIG. 2. The input power is subject to filter module at first so as to filter out external noise signal and obtain pure power. Next, the power enters into power conversion module which determines energy outputted and has function of delivery and transform as the vital part of the switch power. Then, the power enters into rectification and filter module which eliminates noise signal generated by power conversion module or the like. Finally, the power enters into output module and then is supplied to the user. PWM control module controls the entire power to maintain output is always present. Output sampling and feedback module serves to monitor and feedback so as to communicate information. Output sampling and feedback module feeds the monitored information back to the PWM control module, and then the PWM control module controls power conversion to secure stable output of values. The auxiliary power is power supplied to the PWM control module.